A basic multicarrier transmitter is shown in FIG. 1. Serial input data at a rate Mf.sub.s bit/s are grouped into blocks of M bits at a symbol rate of f.sub.s. The M bits are used to modulate N.sub.c carriers (m.sub.n bits for carrier n) which are spaced .DELTA.f.sub.c apart across any usable frequency band. The preferred method of modulation is to use an Inverse Fast Fourier Transform (IFFT), which generates N.sub.samp (preferably equal to an integer power of 2) samples of a transmit signal for each block of M bits.
In the receiver the received signal is demodulated by each of the N.sub.c carriers, and the m.sub.n bits are recovered from each carrier. Again, the preferred method of demodulation is to sample the received signal, group the samples into blocks (preferably comprising a number of samples that is an integer power of 2), and perform a Fast Fourier Transform (FFT).
In such a transmitter the data symbol rate, the signal symbol rate, and the carrier frequency separation, .DELTA.f.sub.c, are all equal, and the receiver uses all N.sub.samp samples to retrieve the data.
A more detailed discussion of the principles of multicarrier transmission and reception is given in J.A.C. Bingham, "Multicarrier Modulation For Data Transmission: An Idea Whose Time has Come", IEEE Commun. Mag., pp 5-14, May, 1990.
FIG. 2(a) shows the connection of a multicarrier transmitter 100 and a basic receiver 1000 through a channel 200. Input serial data are grouped into blocks, converted to a parallel form and encoded by an encoder 120, and modulated by an IFFT operation 130. The digital samples are then converted to analog by a digital-to-analog converter (DAC), low-pass filtered, and sent through a d.c-isolating transformer 140 to produce a transmitted signal which is input to the channel 200. In the receiver, a d.c. isolating transformer, low-pass filter, and analog-to-digital converter 1100 perform complementary operations to produce a digital received signal, followed by an FFT 1010 to produce a frequency-domain signal.
If the amplitude/frequency and delay/frequency responses of the channel are not constant across the whole of the used frequency band (that is, either is distorted), then the received signal will differ from the transmitted signal, and the signals presented to the decoder 1030 will differ from those output from the encoder 120. If the distortion is severe then the data on one carrier (i.e., in one sub-channel) will affect both the signals detected on other carriers (inter-channel interference, or ICI), and the signals detected in that sub-channel in the previous and subsequent symbol periods (inter-symbol interference, or ISI); the decoding device will make incorrect decisions, and data errors will occur. Even if the distortion is insufficient to cause errors by itself, it will reduce the immunity of the signal to any noise o interference that may be added in the channel. Therefore, some form of equalization of the received signal is nearly always necessary.
The most basic form of equalization is performed by the frequency-domain equalize (FEQ) 1020. This compensates individually for the attenuation and delay of each of the sub-channels; if the symbols are very long compared to the duration of the impulse response of the channel this is the only form of equalization that is required. Very long symbols, however, require large memories in both transmitter and receiver, and large amounts of computation in the receiver; they also cause a large delay between input of data to the transmitter and output from the receiver ("latency").
Such memory and computation requirements and such a delay are usually unacceptable, and therefore it is desirable to reduce the symbol length and, equivalently, increase the bandwidth of each of the sub-channels. Then, however, the attenuation and delay of the channel would no longer be constant across the band occupied by each sub-channel, and both ISI and ICI would result. Three methods of reducing this ISI and ICI have been used: (1) staggered modulation, (2) pre-equalization, and (3) use of a waiting, or guard, period before collecting samples for input to the FFT. We will briefly describe each of these, and then introduce the present invention, which uses a combination of the latter two.
1. Staggered Modulation: A form of staggered modulation was first described by B. R. Saltzberg, "Performance of an Efficient Parallel Data Transmission System", IEEE Trans. Commun. Tech., vol. COM-16, pp. 805-811, Dec. 1967. D. Chaffee and M. Mallory improved upon this by encoding and shaping the signals in each of the sub-channels so as to reduce the ICI. Modems manufactured by IMC Corporation used a combination of this method with pre-equalization. The amount of signal processing required in both transmitter and receiver is, however, very large.
2. Pre-equalization: A pre-equalizer 1040 can be inserted before the FFT as shown in FIG. 2(b) in order to compensate, either partially or fully, for the channel distortion. To avoid all ISI and ICI this pre-equalizer would have to perform the same function as the equalizer in a single-carrier modem (e.g., J.A.C. Bingham, The Theory and Practice of Modem Design, John Wilkey & Sons, New York, May 1988.); that is, it would have to make both the attenuation and the delay almost constant. Full pre-equalization has been widely used in single-carrier modems for use on the general switched telephone network (GSTN), where the channel distortion is moderate. For severely distorted channels, however, the amount of signal processing required in the receiver would be prohibitively large. The present inventors know of no multicarrier modems that use only pre-equalization.
3. Waiting period, guard period, or cyclic prefix: If it is judged that the distorted transient response of the channel lasts for less than L sample periods, then the signal symbol length can be increased from N to N+L samples by inserting L extra samples, known as a cyclic prefix, as shown in FIG. 2(c) at 150. If these extra samples are appropriately chosen and if the receiver discards the first L samples of each received symbol, as shown in the figure at 1050, then both ISI and ICI are greatly reduced.
This method was originally described by S. B. Weinstein and P. M. Ebert, "Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform", IEEE Trans. Commun. Tech., vol. COM-19, pp. 628-634, Oct. 1971., and has been used in modems manufactured by Telebit Corporation for use on the GSTN. It can be seen that because only N of each block of N+L samples carry data information, the data throughput efficiency of the method is proportional to N/(N+L). If, because of severe distortion, L is large, then, in order to maintain an acceptable efficiency, N would have to be very large; this would cause a very large (and probably unacceptable) latency.
The most common way of implementing the cyclic prefix is to duplicate the last L samples of the block of N data samples, and add them to the front of the block. Because all of the many (multi) carriers that are used have an integral number of cycles in the N samples, the sum of all the carriers, which comprises the data signal, would have a period of N samples, and there is continuity of signal between the prefix and the data signal.
An equivalent effect could be achieved by adding the extra L samples at the end of the block of N (a cyclic suffix), and using, as with the more conventional cyclic prefix, the last N samples of the composite block for the input to the FFT in the receiver.
4. A guard period combined with adaptive pre-equalization: If the channel distortion is very large, then a good compromise between minimizing the latency and the amount of signal processing and memory required, and maximizing the data throughput efficiency can be achieved by using a short guard period and a short adaptive equalizer (i.e., one with a small number of taps) in the receiver. This combination was proposed in J. S. Chow, J. C. Tu, and J. M. Cioffi, "A Discrete Multitone Transceiver System for HDSL Applications", IEEE JSAC, vol. 9, pp. 895-908, Aug. 1991.